Embodiments of the present invention relate to a semiconductor memory device.
Semiconductor memory devices have been rapidly developed to temporarily or permanently store data therein. The semiconductor memory devices have been widely used in a variety of electronic appliances, electronic devices, and personal portable devices. General semiconductor memory devices can freely read and/or write data, and also easily update old data to new data.
The semiconductor memory device has been developed such that it can store an increasing amount of data therein, operate with a small amount of power, and increase or expedite an operating speed. Although a NOR flash memory device or a NAND flash memory device has been widely used as a non-volatile memory, a conventional flash memory device has a disadvantage that it has a slow operating speed.
In order to overcome the above-mentioned disadvantage, a phase change random access memory (PCRAM) has been developed. The PCRAM changes a resistance value of a material contained in a unit cell to another resistance value using a current signal, stores data corresponding to the changed resistance value, and reads a difference in current between two resistance values. The PCRAM includes a material which causes a temperature-based phase change in a unit cell, and a crystal structure of the material changes to a crystalline state or non-crystalline state (also called an amorphous state) according to a temperature generated by an amount of a current flowing through the material. For example, the material may use germanium antimony tellurium (Ge2Sb2Te5, GST) whose resistance value is changed according to a crystalline or amorphous state.